In a bale press for producing parallelepipedic fodder bales, such as described in U.S. Pat. No. 4,132,164, a fodder charge is compressed by a main plunger driven in reciprocating movement within a compression canal. Fodder slugs are fed into the compression canal through a lateral conduit while the plunger is retracted. The slugs are then compressed by the plunger between the forward face of the plunger and a fodder charge already in the compression canal. The fodder charge already in the compression canal serves as an "anvil" for the plunger. A compression force is exerted on the fodder slug. The compression force is directly related to the force necessary to drive the bale through the compression canal, and is equal to the longitudinal retention forces that result from the friction between the bale and the compression canal inner walls.
In such an operation, however, the friction forces and therefore the retention of the bale may vary greatly. This will result in an unequal compression and non-homogenity of bales.
A device for producing a homogeneous bale by regulating compression force is known from U.S. Pat. No. 4,168,659. The walls of the compression canal in that device are partially formed of pivotable plates pressed against the outer surfaces of the fodder bale. The plates are pushed by variable transversal forces to produce a variable longitudinal retention force. The compression force is a function of the transversal forces. The plunger force is in balance with the force required to slide the bale through the canal. The force required to slide the bale depends on the transversal forces applied by the plates and on the coefficient of friction which relates the transversal or normal forces to the longitudinal force. A dynamic force also exists in such a device due to the acceleration of fodder mass in the canal.
The principal object of a bale press is to optimally compress the fodder according to the user's criteria. A continuous and regular density must be attained, implicitly requiring a continuous and regular force by the compression plunger. During the operation of a bale press many factors may influence the coefficient of friction between the fodder and the compression canal (e.g., humidity, type of product). The forces exerted by the plunger may also vary greatly at a particular work site.
In U.S. Pat. No. 4,168,659, pivotable plates are actuated by hydraulic cylinders. The main plunger comprises a connecting rod eccentricly mounted and cooperating with a piston-cylinder group to provide fluid under pressure to the cylinders and to provide pilot pressure to a regulating valve. The pilot pressure is a function of the compression force applied by the main plunger.
Such an arrangement, however, has several drawbacks. Firstly, the system is incapable of reacting immediately if the compression force is too small. One or more reciprocal plunger strokes are required to increase the pressure in the cylinders of the pivotable plates during which time successive fodder slugs are insufficiently compressed.
Secondly, the cylinder may become excessively discharged if the regulation is poorly adjusted or becomes unadjusted.
Finally, the system suffers from considerable hysteresis leading to relatively long response times because the piston-cylinder group associated with the eccentric mounting of the main plunger connecting rod functions as both a fluid source and a pressure detector. Thus, the actual compression applied by the main plunger can greatly exceed the desired threshold before the spool of the regulating valve responds to discharge the cylinder.